A polymer electrolyte fuel cell has a basic structure arranging a cation-exchange membrane which allows protons to be selectively permeated, catalyst layers which are joined to both sides thereof, and further gas diffusion layers such as carbon paper on their external sides. The above-described catalyst layer mainly includes an anode on which a reaction occurs with hydrogen as an active material and a cathode on which a reaction occurs with oxygen as an active material. When hydrogen and oxygen as the active materials are supplied. to the respective catalyst layers, the reaction of H2→2H++2e− (E0=0 V) occurs on an anode catalyst while the reaction of O2+4H++4e−→2H2O (E0=1.23 V) occurs on a cathode catalyst, and electricity is generated by a resulting potential difference. For efficient electricity generation, a gas migration pathway for supplying a reactant gas which is an active material to the catalyst in the catalyst layer are necessary; a proton-conducting pathway through which protons and electrons generated by the anode are transported to The cathode; and an electron transfer pathway. In general, the catalyst layer is constituted by catalytic metal-supported carbon particles having electrical conductivity and a polymer electrolyte material to satisfy the above-described properties. Gaps between the catalyst particles and the pores of the catalyst particles play the role of the gas migration pathway, and the electron-conducting pathway is formed by contact of the catalyst support particles.
Carbon particles which support a catalytic metal such as platinum or platinum alloy and have electrical conductivity are generally used in an electrode catalyst. Since the catalytic metal such as platinum used in the electrode catalyst is a very expensive material, development of a fuel cell that exhibits excellent electricity generation performance in a small amount of platinum used is crucial for proceeding with practical use of a fuel cell.
In recent years, it has been said that reduction in cost is a crucial condition in the practical use of a fuel cell, and a catalytic metal such as platinum used in an electrode catalyst makes up a very large proportion of the cost.
As a method for improving the utilization rate of a catalytic metal, for example, platinum has been made to be fine particles to increase an exposed surface area. By decreasing the particle diameters of the catalytic metal particles, the utilization rate of the catalytic metal can be raised, since the exposed surface area of the catalytic metal is increased even when the amount of the catalytic metal used is the same. However, it is difficult to disperse the catalytic metal such as platinum or platinum alloy as fine particles on a carbon surface, the particles very easily agglomerate even when the particles can be made fine, and, therefore, the catalyst particles are easily enlarged by driving a fuel cell. Therefore, regarding particle diameter of a platinum particle supported on a carbon particle, in general, platinum having the particle diameter of typically around 3 nm is often supported.
As a fuel cell electrode catalyst, there has been proposed an electrode catalyst characterized in that it includes a core-shell structure with a core unit including a noble or transition metal and a shell unit including a noble metal-containing layer which is formed on the periphery thereof and of which the composition is different from that of the core unit. In a core-shell-type catalytic metal fine particle, since a highly active catalytic metal can be disposed only on the surface (shell) thereof, the exposed surface area per unit weight of the highly active catalytic metal is large. Therefore, the electrode catalyst having the core-shell structure is excellent in the utilization rate of a catalytic metal contributing to the activation of an electrode reaction and enables the amount of the catalytic metal used to be reduced.
For the production of the core-shell catalyst, synthesis can be performed by electrochemical production methods as exhibited in Patent Literature 1 described below and the like. When the techniques are used, a material having an ideal core-shell structure with the high coating degree of a shell can be produced.